Corona discharge charging of particles wherein a porous insulator is disposed between the corona electrodes



June 30, 1970 M. MICHALCHIK 3 5 CORONA DISCHARGE CHARGING 0F PARTICLESWHEREIN A POROUS INSULATOR IS DISPOSED BETWEEN THE CORONA ELECTRODESFiled Jan. 2, 1968 lxvavrola. MMMEL M0944 ov/K United States Patent US.Cl. 317-3 15 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method forelectrostatically charging particles. Spaced apart electrodes adaptableto set up corona discharge therebetween are provided and porousinsulator is interposed between said electrodes, insulator being ofsufficient area to effectively separate corona charge distribution inaerial portions adjacent the opposite surfaces of insulator.

This invention relates to electrostatically charged particles, and inparticular to a method and apparatus for applying a uniformelectrostatic charge to particles and the like.

In the past, various methods and means have been utilized toelectrostatically charge particles. Some prior art methods utilized thephenomena of triboelectrification to electrostatically charge theparticles; these methods usually entailed the rubbing of the particlesto be charged against a surface or other particles of a dissimilarsubstance so that each becomes oppositely electrified.

In one prior art method, triboelectrification is carried out by mixingthe particles to be charged with a slightly more coarsely-dividedmaterial commonly referred to as a carrier. Upon agitation of thismixture, the frictional contact between the particles and the carriercharge the particles due to the triboelectric effect. This method,however, has been found undesirable for many reasons, notably the lackof uniformity of the charge placed on the particles, the difficulty ofsustaining the charging process for appreicable periods of time, and thedifficulty of separating the particles from the carrier after theparticles have been charged.

In another prior art method, triboelectrification is carried out bymixing the particles in a stream of gas and forcing the resultantmixture through a capillary structure having interior walls constructedof poorly conducting material so that the particles rub against the sidewalls and acquire an electrical charge. This method has also been foundunsatisfactory since the particles are not uniformly charged and sinceno means are available to efiiciently and properly compensate for thevarious particle sizes and types utilized and the various conditions ofhumidity normally attendant such processes.

In comparison to the above described methods which utilize thetriboelectric effect, other prior methods electrostatically chargeparticles 'by passing the same through a corona discharge system. Acorona discharge system usually consists of two spaced apart electrodes,at least one of which may be a very thin wire. Upon the application of avery high voltage difference between the spaced apart electrodes, acorona discharge is set up between the electrodes due to the resultantionization of the gaseous medium surrounding the electrodes. Thus, bypassing the particles through this discharge, the particles areelectrostatically charged by the medium.

The corona method of electrostatically charging particles has been founddesirable since no carriers are utilized and this obviates thedifficulties attendant the men 3"5l8,488 Patented June 30, 1970 tionedsystems which utilized carriers and since the corona discharge densitycan be eifectively regulated in order to compensate for variation inparticle size and type as well as changing conditions of humidity. Thecorona discharge method as practiced heretofore, however, like the othermentioned prior art methods, failed to adequately and uniformly chargethe particles as they passed through the corona discharge. Moreover, ithas been found that undesirable particle precipitation and clumpingoccur about the corona electrodes.

The present invention substantially alleviates the abovernentioneddifiiculties of the prior art methods and provides a method andapparatus which applies a substantially uniform charge to particleswithout the use of carriers and yet permits compensation for varyingparticle size and type as well as changing humidity conditions. Thus, inaccordance with the present invention, the beneficial features attendantprior art corona discharge systems are retained but substantiallyuniform charge distribution is achieved.

In a corona discharge system, each of the electrodes producing thecorona discharge charges its surrounding gaseous medium withelectrically opposite charges, and there is thus produced a mixture ofboth positive and negative charges in the corona discharge zone. Thischarge mixture reduces the uniformity of the charge applied to theparticles in the zone, and also is the main contributing factor whichcauses particle precipitation and clumping. These latter factors areproblems with many prior art corona devices.

In accordance with the present invention, a porous in sulator isinterposed between the electrodes producing the corona discharge. It isselected to be of sufficient area to effectively separate the coronacharge distribution in the aerial portions adjacent its oppositesurfaces. Thus, the insulator serves to separate the above notedopposite charges produced by each of the corona electrodes and also actsto neutralize any charge attempting to permeate it so that as a result azone of uniformly charged gas is produced on the opposite sides of theinsulator. Accordingly, by passing the particles to be charged in astream of air or neutral gas through one zone of the segregated coronadischarge, the particles will acquire a substantially uniform charge.

According to another aspect of the present invention, the insulator maybe positioned in the corona discharge closer to one electrode than theother in order to increase the working volume through which theparticles to be charged may pass and acquire the desired charge. Also,in order to prevent clogging of the pores of the insulator, one sidethereof is preferably maintained at a higher pressure than the other.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the present invention that will be describedhereinafter and which will form the subject matter of the claimsappended hereto. Those skilled in the art will appreciate that theconception upon which this disclosure is based may readily be utilizedas a basis for the designing of other structures for carrying out theseveral purposes of the invention. It is important, therefore, that theclaims be regarded as including such equivalent constructions as do notdepart from the spirit and scope of the invention.

Certain specific applications of the invention have been chosen forpurposes of illustration and description, but the invention should notbe limited thereto. These specific applications are shown in theaccompanying drawing and p CC are described in the following portions ofthe specification.

In the drawing:

FIG. 1 is a diagrammatic illustration of one suitable form of thepresent invention;

FIG. 2 is a diagrammatic illustration of a tubular em.- bodiment of thepresent invention.

Referring to FIG. 1, the present invention comprises two spaced apartelectrodes, one being a corona wire electrode and the other being aplate electrode 12 of opposite polarity. A porous insulator 11 isinterposed between electrodes 10 and 12; the porous insulator maycomprise sintered glass with openings ranging from 230 microns. In thepreferred embodiment, the insulator 11 is situated further from thecorona electrode 10 than from the electrode of opposite polarity 12 soas to increase the working volume through which the particles pass. Asshown in FIG. 1, the porous insulator 11 is preferably separated fromelectrode 12 by insulating members 13. At a small distance above coronaelectrode 10, a protective shield 15 is provided; the shield providesprotection from the corona wire 10 and may form a closure for theparticles as described hereinafter.

In order to electrically charge the particles, a powder cloud of theparticles is generated by any conventional means (not shown). Thesuspended particles are then passed between the corona electrode 10 andthe porous insulator 11 in the direction indicated by the arrows shownin FIG. 1. Simultaneously, a direct current voltage generated by anyconventional means (not shown), is applied between the electrodes 10 and12 so as to ionize the surrounding air medium and set up a coronadischarge. The effect of the porous insulator 11 is to separate anyoppositely charged gaseous particles created by the electrode 12 fromthe charged gaseous particles created by the corona electrode 10. Thus,a zone designated by numeral 16 of uniform charge is created and as theparticles pass through zone 16, they acquire a substantially uniformcharge.

In order to compensate for the various physical properties of theparticles being charged and changing humidity conditions, the amount ofvoltage applied to corona electrode 10 and the electrode 12 may beregulated.

In order to prevent any clogging of the pores in insulator 11 by thetoner particles, a slight pressure is maintained in zone 14. Thepressure is developed by any conventional means (not shown) and directedinto zone 14 by piping (not shown).

FIG. 2 shows a alternate embodiment of the present invention. As showntherein, the corona electrode 20 is completely surrounded by the porousinsulator 21 which is, in turn, surrounded by a metal shield 22 servingas the opposing electrode. The porous insulator 21 is separated fromelectrode 22 by stand off insulating members 23. In this embodiment, theparticles are blown while suspended in air through zone 26 whilepressure is maintained in zone 24 to keep the toner powder from cloggingthe porous insulator 21.

A preferred material suitable for use as a corona electrode 10 or 20 isa very thin molybdenum and gold coated copper wire; those skilled in theart, however, will appreciate that other materials are also acceptablefor use as corona electrodes. It should also be noted that the opposingelectrode 12 or 22 may take several forms depending on the size andproposed use of the apparatus. Thus, the electrode 12 may be a metalplate, or wires placed in succession while electrode 22 may be a metaltube or the like. The only limiting factor is that these electrodes mustbe of an opposite polarity than the corona electrode 10 or 20.

The present invention has a multiplicity of applications such as theelectrostatic charging of toner utilized in electrostatic printing andreproduction techniques, the separation and classification of particlematerials, and antiair pollution devices which electrostatically chargeparticles in smoke and air and precipitate the particles in a cloudchamber.

In one embodiment of the present invention adaptable forelectrostatically charging toner particles for electrostaticreproduction, good results were obtained by separating the electrodes 10or 20 from electrodes 12 or 22 between 0.5 and 2.0 inches whilemaintaining the distance between the corona electrode 10 or 20 andporous insulator 11 between 0.6 and 0.9 of the distance between theelectrodes 10 or 20- and 12 or 22, respectively.

There exists in the art certain types of electrostatic printers whereina printing drum is rotated in such a manner as to require intermittentdeposition of the toner particles on the electrostatic latent imagethereon; the latter is usually utilized in color reproductiontechniques. The present invention may advantageously be utilized in suchprinters since by pulsing the corona voltage in conjunction with pulsingthe toner flow, the intermittent toner deposition can be accomplished inone simple operation.

Thus, it may be seen from the above that the present invention overcomesmost of the difiiculties attendant prior art electrostatic chargingdevices and provides a method and apparatus which applies asubstantially uniform charge to particles without the use of carrierswhile permitting compensation for variation in particle size and type aswell as changing humidity conditions.

What is claimed is:

1. An apparatus for electrically charging particles comprising spacedapart electrodes adaptable to set up a corona discharge therebetween anda porous insulator interposed between said electrodes, said insulatorbeing of sufiicient area to effectively separate the corona chargedistributions in the aerial portions adjacent the opposite surfaces ofsaid insulator.

2. The apparatus according to claim 1 wherein said insulator isinterposed between said spaced apart electrodes in a position closer toone electrode than the other.

3. The apparatus according to claim 1 wherein one of said spaced apartelectrodes includes at least one corona wire electrode and the other ofsaid spaced apart electrodes includes a metal plate electrode.

4. The apparatus of claim 3 wherein said metal plate electrode is acylindrically shaped tube and said corona wire is centrally locatedtherein and extends along the longitudinal axis thereof.

5. The apparatus according to claim 1 wherein both of said spaced apartelectrodes includes at least one corona wire.

6. The apparatus of claim 1 wherein said porous insulator comprisessintered glass.

7. An apparatus for electrical charging particles comprising meansdefining a chamber, spaced apart electrodes positioned in said chamberand adaptable to set up a corona discharge therebetween, a porousinsulator interposed between said electrodes, said insulator dividingsaid chamber and efiectively segregating the corona charge distributionin the chamber portions adjacent the opposite surfaces of said insulatorwhereby a substantially uniform charge distribution is produced in atleast one of said portions.

8. The apparatus according to claim 7 wherein the portion of saidchamber providing said uniform charge distribution includes particleinput and output means, and a particle cloud generator means isconnected to said input means for directing particles through saiduniform charge distribution to said output means.

9. The apparatus according to claim 8 wherein the other of said portionsincludes means for maintaining the pressure therein higher than that inthe said portion providing said uniform charge distribution.

10. The apparatus according to claim 9 wherein said insulator comprisessintered glass.

11. The apparatus accordina to claim 7 wherein said insulator isinterposed between said spaced apart electrodes in a position closer toone electrode than the other.

12. The apparatus according to claim 7 wherein one of said spaced apartelectrodes includes a metal cylinder and the other electrode includes acentrally located thin corona Wire extending along the longitudinal axisof said metal cylinder.

13. The apparatus according to claim 12 wherein said metal cylinderdefines said chamber.

14. The apparatus according to claim 13 wherein said porous insulator iscylindrical in shape and is interposed concentric with said metalcylinder.

15. A process for electrically charging particles comprising passingsaid particles through a segregated portion UNITED STATES PATENTS 1/1969 Topper et al 317-262 WALTER STOLWEIN, Primary Examiner 10 A. L.BIRCH, Assistant Examiner US. Cl. X.R.

